Donald Trump is "morally unfit" to be president, former FBI chief James Comey asserted today as he claimed "it's possible" that Moscow may have information on him that could be used to compromise him.

Comey, who was unceremoniously fired by the US president in May last year, in a wide-ranging interview to ABC News also warned that if Trump ever tries to sack special counsel Robert Mueller, then it would be his "most serious attack yet on the rule of law" and causing "tremendous damage" to the country's institutions.

Mueller is investigating alleged Russian interference in the 2016 US election and any potential ties between Trump campaign and Russia. Trump has repeatedly called the investigation a witch hunt and denied any collusion.

The former Federal Bureau of Investigation director's blunt comments came as the 57-year-old lawyer kicked off a high-profile tour to promote the release of his new book, "A Higher Loyalty."

Asked if he thinks the President has been compromised by the Russians, Comey said, "I think it's possible, I don't know," Comey said.

"I wish I wasn't saying it, but it's the truth. It always struck me, and still strikes me, as unlikely, and I would have been able to say with high confidence about any other president I dealt with, but I can't, it's possible," he said.

Comey expanded on that statement in an interview USA Today published today.

"There's a non-zero possibility that the Russians have some, some sway over him that is rooted in his personal experience, and I don't know whether that's the business about the activity in a Moscow hotel room or finances or something else," Comey said.

Trump, 71, has said the" badly reviewed book" raises "big questions". He also suggested Comey should be imprisoned, and in recent days began referring to him as a "slime ball".

In the ABC News interview, Comey offered a scathing judgment about whether Trump is qualified to serve as president, saying that he believes Trump is "morally unfit" to serve in the nation's highest office.

Comey also said, "I don't buy this stuff about him being mentally incompetent or early stages of dementia."

"Our president must embody respect and adhere to the values that are at the core of this country. The most important being truth. This president is not able to do that," Comey said.

The former FBI director went on to say that a person "who talks about and treats women like they're pieces of meat, who lies, constantly, about matters big and small, and insists that the American people believe it -- that person is not fit to be president of the United States on moral grounds."

During the interview, Comey repeated his assertion that Trump told him that he expected loyalty and pressured him to drop an investigation into Michael Flynn, the former national security adviser.

Flynn has since pleaded guilty to lying to the FBI and is now cooperating with the special counsel.

Trump has denied asking Comey to stop investigating Flynn and insisted he never asked for loyalty.

When Comey testified before the Senate Intelligence Committee last year, he said that he did not know if Trump obstructed justice, adding, "that's Bob Mueller's job to sort that out."

In his book, Comey deferred to the Mueller investigation on the question of obstruction.

"I presume that Bob Mueller's team is investigating ... whether or not urging me to back the FBI off our investigation of his national security adviser and in firing me, President Trump was attempting to obstruct justice, which is a federal crime," Comey writes. "It's certainly possible."

After the interview aired, Trump's party - via the Republican National Committee - released a statement saying Comey's publicity tour for his new book showed "his true higher loyalty is to himself".

Comey served as the seventh director of the FBI from September 4, 2013, until he was fired on May 9, 2017.